Since Andre Geim and Konstanin Novoselof from University of Manchester in UK successfully stripped pyrolytic graphite out and observed graphene in 2004 (Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, V; Dubonos, S. V.; Grigorieva I. V.; Firsov, A. A. Science 2004, 306, 666-9), new carbon materials has been remaining a hot topic in relevant areas. The success of stripping graphene out breaks the prediction about thermal instability of two-dimensional crystal theoretically, and brings about possibilities for more investigations and explorations in new fields.
Perfect graphene is supposed to own ideal two-dimensional structure, which consists of hexagonal lattice. Every single carbon atom is combined with other three carbon atoms by σ bond in the direction of lattice plane, and non-bonding electrons serves as π electrons, forming π orbit system vertical to the lattice plane. As the π electron could move randomly in the plane-graphene can sustain electric current densities six orders higher than copper. Meanwhile, Graphene shows record thermal conductivity. The thermal conductivity of pure graphene could reach 2000-4000 W·m−1·K−1, and also has excellent strength and large surface area. Besides, the special structure of graphene provides unique energy band structure and enables it with half integer quantum hall effect and perfect tunneling effect, as well as electrical conductivity that would never fade away. The special characteristics mentioned above guarantee graphene a promising prospect of application in fields of materials and electronic circuits.
There're two traditional ways to synthesize graphene, which are physical method and chemical method respectively. Properties of graphene obtained through the two methods are different from each other. Physical methods include mechanical stripping, electric arc discharge, ultrasonic dispersion etc. Graphene layers obtained through physical methods are comparatively intact, but there're problems like low productivity, uncertainty of quality, command for special equipment and high cost. While chemical methods include bottom up organic synthesis, oxidation-reduction process, solvothermal synthesis and chemical vapor deposition. Equipment and raw materials are strictly required for organic synthesis method, so it's difficult to achieve mass production in this way. Production quality isn't stable for solvothermal method, thus the average quality is poor. Chemical vapor deposition method costs too high and cannot achieve scale production. Among all those methods, only oxidation-reduction process can work without special equipment, and quality of graphene obtained through this method is stable. Thus it's the most suitable way for industrialized production.
During the oxidation-reduction process for graphene preparation, intermediates of graphene oxide has been involved. The intermediates are supposed to go through intercalation by strong acid. Then the intercalated graphene goes through deep oxidation by strong oxidant to form large quantity of carboxyl and carbonyl groups around the graphene layers, and then form large quantity of hydroxyl and epoxy groups within the graphene layers. After ultrasonic exfoliation, we can obtain graphene oxide with a certain degree of dispersion. A large amount of strong acid and strong oxidant is used in this step, meanwhile the heat release is quite serious. As a result, equipments for graphene preparation must meet strict standards, and it's difficult to achieve mass production. Besides, during the oxidation-reduction process for graphene preparation, high quality graphite is required, and we usually use crystalline flake graphite of higher price and purity as main material, which increases the cost of graphene production further. Production cost for high quality single-layer graphene is especially high. Therefore, if we could develop raw materials at low cost for graphene or graphene oxide, and design a reasonable manufacturing technique based on the material, we would be able to lower the production difficulty and cost effectively and make it possible to put graphene into mass production.
Coal serves as a traditional material in chemical engineering and energy fields. Chemical composition of coal can be regarded as macromolecule polymerized by large amounts of condensed rings with different condensation degrees and different groups. There're a few aliphatic rings and heterocyclic rings apart from the condensed rings, as well as some carbon groups like alkyl. Except for carbon group, there're lots of alkoxy, hydroxyl, carboxyl and sulfhydryl groups in coal, and some complex groups (mainly oxygen-containing groups), containing oxygen, suffer and nitrogen like amidogen. Therefore, we can classify coals into different categories according to the ratio of carbon content in carbon groups and oxygen content in oxygen-containing groups. Anthracite, with highest degree of coalification, one kind of coal with highest carbon content. In general, the value of carbon content could reach 90%. The number of aromatic nucleus in basic anthracite structures increases dramatically which tends to show graphite structure gradually, and it was obviously observed in models of Larsen (Cooper, B. R., Petrakis, L. Eds., American Institute of Physics: New York, 66-81 (1981)). Theoretically, this graphite-like structure can effectively serve as precursor of graphene and graphene oxide synthesis. Coal resources are abundant in our country and thus the price is low. If we can use anthracite as raw material of graphene, the production cost can be significantly reduced. Besides, there're always residues of hydroxy, carbonyl and carboxyl groups in anthracite, which are more favorable for formation of graphene oxide compared to graphite.